The cellular mechanism that maintains the postnatal thymic epithelial (TE) compartment is currently unknown. Precedent exits for two possible models: the TE may be maintained by a stem/progenitor cell mechanism, as in the intestine, or by mitosis of terminally differentiated cells, as in the liver. Some currently existing data have been interpreted to support a stem cell mechanism, however, this conclusion is inferred rather than direct. Unequivocal discrimination between these possibilities is important, since development of effective strategies aimed at thymus regeneration will depend on a clear understanding of the target cell population(s) for therapeutic intervention. Previous work from this laboratory has identified and characterized the fetal epithelial progenitor cell (TEPC) population from which the thymus arises [1-3]. This population corresponds to a common thymic epithelial progenitor cell that can generate all TEC subtypes, and is identified by mAb MTS24. A rare population of MTS24+ TEC is present in the medulla of the postnatal thymus. Our preliminary data indicate a lineal relationship between fetal and adult MTS24+ TEC and demonstrate that adult MTS24+ TEC retain BrdU, a characteristic of some stem cell populations. Postnatal MTS24+ cells may therefore be TESC; alternatively, they may be a quiescent, terminally differentiated TEC population. Pax1+ cellsjn the thymic cortex, and cytokeratin (K)5+8+ cells at the cortico-medullary junction and in the cortex have also been suggested as candidate TESC. Genetic and cellular approaches now exist that permit rigorous evaluation of the contribution of these populations to homeostatic maintenance of the TE. This project will therefore determine the cellular mechanism of thymic homeostasis, via candidate cell and unbiased approaches. RELEVANCE (See instructions): The proposed experiments will provide essential information regarding the cellular mechanisms that maintain the postnatal TE throughout life and the impact of disrupting these mechanisms on homeostasis and involution. They will therefore inform strategies aimed both at inducing thymus regeneration in situ, and at boosting thymus function via cell therapy using cells expanded or generated in vivo.